Conversion of natural gas to liquid hydrocarbons (“Gas To Liquids” or “GTL” process) is based on a 3 step procedure consisting of: 1) synthesis gas production; 2) synthesis gas conversion by Fischer-Tropsch (“FT”) synthesis; and 3) upgrading of FT products (wax and naphtha/distillates) to final products.
The Fischer-Tropsch reaction for conversion of synthesis gas, a mixture of CO and hydrogen, possibly also containing essentially inert components like CO2, nitrogen and methane, is commercially operated over catalysts containing the active metals Fe or Co. Iron catalysts are best suited for synthesis gas with low H2/CO ratios (<1.2), e.g. synthesis gas produced from coal or other heavy hydrocarbon feedstock, where this ratio is considerably lower than the consumption ratio of the FT-reaction (2.0-2.1).
To achieve sufficient catalytic activity, it is customary to disperse a catalytically active metal on a catalyst carrier, often referred to as the support material. In this way, a larger portion of the metal is exposed as surface atoms where the reaction can take place. Typically, support materials are alumina, silica and titania based. In addition, different promoters are also added to further increase catalytic activity, and typical promoters may be rhenium, ruthenium and platinum. The F-T process can be carried out either in a fixed bed reactor or a slurry bed reactor. In case of a slurry bed process, the catalyst particles are suspended in oil with gaseous reactants being bubbled into the reactor. For either process to be economically viable, the catalyst must exhibit good performance for a long period of time without significant loss in catalytic activity. Typically, catalyst deactivates because of one or more of the following issues: (a) poisoning of the active catalytic metal (e.g. cobalt), (b) loss of catalytic metal surface area (e.g. via sintering), (c) loss of active metal species due to reaction with support, and (d) attrition.
The attrition of the catalyst, i.e. issue (d) above, is primarily dependent on the strength of the support for the catalytically active metal. Using slurry bed catalysts are subjected to a number of collisions either with other particles or with the reactor walls. This causes the catalyst particles to “attrit” or break into smaller particles. Smaller particles are not retained in the reactor, and as a result the activity declines absent continuous addition of fresh catalyst. In order to enhance performance of the catalyst and to improve the catalyst life, a support must therefore exhibit high attrition resistance.
High surface area alumina is commonly used as a catalyst support for F-T. Supports having high surface area provide the necessary support for dispersing catalytic sites throughout the catalyst. High surface area aluminas are conventionally prepared by calcining an aluminum hydroxide composition such as boehmite. Calcined, high surface area alumina per se, however, does not exhibit good attrition resistance. Indeed, it is also largely believed that aluminas after calcination cannot be easily bound into hard particles. Hence, there is a tendency to use boehmite aluminas as support precursors, which are slurried in water and “peptized” in the presence of an acid such as nitric or hydrochloric acid, followed by drying and calcinations to give attrition resistant particles. This alternative presents its own problem because these peptized boehmitic aluminas slurries gel at high solids content and need to be diluted before drying and calcination. Processing the alumina at high solids content is desirable not only to get high production rates, but also to yield a strong particle of desired particle size upon spray drying.
In addition, and as reference with respect to issue (c) above, high surface area alumina supports react with active metal precursor of cobalt to form Co-aluminate spinel upon calcination. This transforms the active Co metal to “inactive” spinel Co-aluminate and thus decreases the catalyst activity.
In order to prevent Co-aluminate spinel formulation, divalent metals like Ni, Zn, and Mg can be added to an alumina support to form the spinel phase “a priori” and thus prevent the formation of inactive Co-aluminate. The divalent metal aluminate spinels are formed upon high temperature calcinations above 650° C. Such spinel materials do not exhibit high strength, however, and can easily break into smaller particles. In other words, such spinel phase-based particles generally do not have sufficient attrition resistance.
It has been shown that if the spinels compositions are calcined at very high temperatures, in excess of 1100° C., the attrition resistance improves significantly (see WO 2005/072866 A1 or US 2007/0161714). In addition to requiring high calcination temperatures, it is also apparent that high levels of divalent metals are needed to attain the good attrition resistance. Typically, the divalent compound is in excess of 10 wt % (as metal) in loading.
It has also been shown that, as a result of high temperature calcinations, the support pore diameter shifts to high pore modes. Catalysts made from these high temperature calcined spinel supports therefore have high selectivity to high hydrocarbons in addition to the aforementioned attrition resistance. The practical use of these supports, however, is limited due to expensive processing steps, and large amounts of expensive divalent metal compounds added as dopants. Furthermore, large amount of divalent dopant compounds poses the risk of leaching out of the spinel structure and adversely affecting the catalyst activity.